The present invention relates to a fuel cell system.
A fuel cell has a stack structure with a plurality of stacked power generation elements as base units of power generation (hereinafter also called “cells”). A fuel cell system configured to include such a fuel cell is generally known. JP 201.1-034837A describes a method of starting a fuel cell system in a subzero environment.
At a start of the fuel cell system like the fuel cell system described in JP 2011-034837A, in the case of detection of a decrease in voltage of the fuel cell, the fuel cell system provides an output limit of the fuel cell, in order to suppress deterioration of the cells of the fuel cell. More specifically, the fuel cell system provides an upper limit of the electric current required for the fuel cell.
The cell is more likely to deteriorate by a decrease of the voltage at the higher temperature of the cell. Accordingly the fuel cell system increases the output limit amount of the fuel cell with an increase in temperature of the fuel cell. The temperature of a cooling medium used to cool down the fuel cell is used as an index indicating the temperature of the fuel cell. The temperature of the cooling medium is readily obtained by measuring the temperature of the cooling medium that is discharged from the fuel cell, with a temperature sensor. In the case of detection of a decrease in voltage of the fuel cell, the fuel cell system may provide an output limit of the fuel cell with the magnitude of an output limit amount according to the temperature of the cooling medium.
In the fuel cell having the plurality of cells, the temperatures of the respective cells of the fuel cell are not uniform. For example, end-portion cells that are located in an end portion of the stack structure are more likely to release heat and have the poorer temperature rise performance, so as to have the lower temperature, compared with center-portion cells that are located in a center portion of the stack structure. The temperature of the cooling medium is, however, the index indicating the temperature of the entire fuel cell. The output limit of the fuel cell using the output limit amount according to the temperature of the cooling medium accordingly results in providing an excessive output limit especially for the end-portion cells of the fuel cell. The excessive output limit is likely to reduce the amount of heat generated by the respective cells of the fuel cell and cause deterioration of the startability of the fuel cell.
There is accordingly a need for a fuel cell system that readily provides an output limit of a fuel cell by using temperature of a cooling medium, while improving the startability of the fuel cell.